This invention relates to an optical fiber amplifier circuit comprising a plurality of excitation light sources. The optical fiber amplifier circuit is particularly useful in an optical fiber communication system.
Recently, attention is paid to an optical fiber amplifier which uses an optical fiber doped with a rare-earth element, such as Neodymium or Erbium. Such an optical fiber amplifier is described in an article which is published by Kazunori Suzuki under the title of "Optical fiber amplifier module" in "The Journal of the Institute of Electronics, Information and Communication Engineers", pages 221-224, Vol. 74, No. 3, March 1990.
Such an optical fiber amplifier module comprises an Er(Erbium)-doped optical fiber, first and second excitation light sources, such as laser diodes, and first and second fiber couplers which are coupled to input and output sides of the Er-doped optical fiber, respectively. The first and the second excitation light sources generate first and second excitation lights, respectively. Each of the first and the second fiber couplers is of a wavelength division multiplexing type. The first fiber coupler receives input light and the first excitation light and multiplexes the input light and the first excitation light into first multiplexed light. The first multiplexed light is supplied to the Er-doped optical fiber. The Er-doped optical fiber absorbs the first excitation light and amplifies the input light and delivers amplified light to the second fiber coupler. The second fiber coupler multiplexes the amplified light and the second excitation light and supplies the second excitation light to the Er-doped optical fiber for obtaining high gain and high power amplification. The second fiber coupler delivers the amplified light to an output port as amplified output light.
It is desirable that the number of the excitation light sources is greater than two in order to improve amplification characteristic of the optical fiber amplifier module. However, if the optical fiber amplifier module comprises a plurality of excitation light sources which are greater than two in number, it is hard to control the respective excitation light sources. In other words, a control method of the plurality of excitation light sources is not yet established.